The present invention relates generally to auditory prostheses, but will be principally described in relation to multi-channel cochlear implants. Such an implant conventionally consists of three components--an implanted electrode array, an implanted receiver/stimulator unit (RSU) and an externally worn speech processor. The speech processor receives sound signals, for example via a microphone, processes them so as to produce a set of signals corresponding to stimuli, then communicates these signals to the RSU. Communication between the speech processor and the RSU may be by an inductive link, a direct cable, or any other suitable means. The RSU, in accordance with the received signals, provides electrical stimulation signals to the electrode array.
For each implantee, it is necessary to set the dynamic range of the stimulus pulses presented by the electrode array in order to optimally and comfortably enhance speech perception by the implantee. The dynamic range is generally set between two parameters--the threshold level (T), being the minimum amount of electrical stimulation that is required to elicit a perceived sound from the implantee, and the comfort (C) level, defined as the maximum amount of electrical stimulation which can be applied before the patient reports discomfort. The T and C levels typically vary for each channel in a multichannel implant.
Conventional setting of the dynamic range uses an elaborate audiometric process, heavily reliant upon patient responses, to set T & C levels. A particular difficulty exists in relation to children, who are often unable to provide meaningful indications as to their perceptions and responses to various stimuli. Moreover, it would be desirable to allow patients to reset the dynamic range using an automatic process, as required, so that physiological variations in their perception can be accounted for. Some of these variations are routine--for example, commonly dynamic range will vary during a woman's menstrual cycle, or may vary with medication or illness. The present system for dynamic range setting requires the services of a trained audiologist in a clinic, and hence cannot provide routine resetting when required by the patient.
Various workers have examined the use of either the stapedius reflex or various evoked action potentials with a view to objectively setting speech processors.
The stapedius muscle, when contracted, acts as a dampening mechanism on the ossicular chain within the ear. In the normally functioning ear, contraction of the stapedius attenuates the vibration transmitted through the malleus, incus and stapes to the oval window, so as to prevent overstimulation of the auditory system. A survey of the prior art shows that the general approach to measuring the stapedius reflex has been to use an acoustic probe, placed in the ear contralateral to the applied stimulation, in order to measure the muscle's response via the mechanical impedance of the tympanic membrane. This approach allows for accurate measurement of the response of the stapedius but is not appropriate for implementation in an implanted device.
For example, Battmer et al. (Electrically Elicited Stapedius Reflex in Cochlear Implant Patients--Ear and Hearing Vol. 11, No. 5, 1990), investigated the use of stapedius reflex evaluations for objective setting of cochlear implant speech processors. In contrast to the present invention they recorded the stapedius muscle's response to electrical stimulation of the cochlea by means of a contralateral acoustic impedance meter. The level of contraction of the stapedius muscle was used to determine both the T and C level.
In a paper by Stephan et al ("Acoustic Reflex in Patients with Cochlear Implants" American Journal of Otology Vol 12, Supplement 1991) the authors indicated that psychoacoustic tests relying on evoked auditory responses and electrically elicited acoustic stapedius reflexes were of use in setting the patient's speech processor. However, that paper taught that acoustic reflex testing using contralateral detection is recommended over electrophysiologic methods because of the difficulties associated with overcoming the difficulties presented by artefacts in the electrophysiological methods.
In a paper by Jerger et al, in Ear and Hearing, vol 9, No 1 (1988), entitled "Prediction of dynamic range for the stapedius reflex in cochlear implant patients", amplitude growth functions for an electrically-elicited stapedius reflex were compared with behavioural estimates of dynamic range. This paper concluded that comfort levels are typically greater than or equal to the saturation or plateau, level of stapedius response. The stapedius reflex, whilst electrically elicited, was measured using an external acoustic probe arrangement.
In a 1990 paper by Shallop et al ("Electrically Evoked Auditory Brainstorm Responses (EABR) and Middle Latency Responses (EMLR) Obtained from Patients with the Nucleus Multi-Channel Cochlear Implant" Ear and Hearing Vol 11, No. 1) the technique of using EABR measurements to set dynamic range was investigated. The author's conclusion was that EABR and EMLR measurements correlate better with comfort levels than with threshold levels. In a paper by Shallop et al ("Prediction of Behavioural Threshold and Comfort Values for Nucleus 22 Channel Implant Patients from Electrical Auditory Brain Stem Response Test Results", Annals of Otology, Rhinology, & Laryngology, vol 100, No 11 (Nov 91)) the authors again discussed and investigated prediction of behavioural threshold and comfort level values using EABR procedures. In both of these papers, the neural response is obtained via a second monitoring mechanism not associated with the implant, and later correlated. The authors state that they are "cautious" about inferring T and C levels to be expected from speech from EABR and EMLR recordings.
None of these papers disclose an arrangement, in which the parameter is electrically measured, and this measurement is directly input to the receiver stimulator unit for use in deriving dynamic range. Moreover, these papers do not disclose any arrangement which could automatically adjust dynamic range without input from skilled personnel.
It is an object of the present invention to provide an arrangement in which at least one of the dynamic range parameters are automatically derived and processed, without the necessity for the implantee's perceptions to be subjectively assessed. It is a further object of the present invention to provide an auditory prosthesis arrangement in which the dynamic range parameters are able to be automatically reset by the implantee without the need for specialised external equipment and personnel.